Method of bonding a semiconductor die to a package using a gold/silicon preform and cooling the die and package through a monotonically decreasing temperature sequence

ABSTRACT

An Au/si die attach method for attaching a die to a package including preheating the package, melting an Au/Si preform in the package cavity, scratching the die onto the package cavity to form a die attach bond, and gradually cooling an Au/Si die bond by reducing heat supplied to the package, so as to cool the package through a monotonically decreasing sequence of temperatures, wherein the package is maintained for a predetermined period of time at each temperature in the sequence. 
     A heating block with segments supplies a decreasing amount of heat to the packages to let the die attached bond gradually cool. The packages are kept on top of the segments of the heating block for a predetermined period of time and each segment is heated to a specific temperature. The gradual cooling of the die attach bond decreases the thermal resistance and prevents the creation of voids in the die attach bond.

BACKGROUND OF THE INVENTION

The present invention relates to methods and apparatus for bonding asilicon die to a package in the construction of a semiconductor chip.Conventional die attach bonding methods include gold/silicon die attachbonding. Since in Au/Si die attach bonding the die is attached to thepackage at temperatures above 400° C., the Au/Si die attach method issuitable for bonding silicon dies to ceramic rather than plasticpackages. One limitation of the prior art gold/silicon die attachbonding methods is that these methods must use a die of the size 62.5Kmil² or smaller in order to meet the die attach bond's thermalresistance requirements. In the prior art method, the preform isunevenly heated and cooled and the die bond of a large die cools tooquickly. Therefore, when the prior art method is used for a large die,stresses and voids are formed in the die bond and the die bond has ahigh thermal resistance.

The prior art Au/Si die attach method uses an Au/Si preform to attachthe die to the package. The package cavity is heated up to a temperatureabove the Au/Si eutectic point so that the Au/Si preform will melt. Thepreform is put into the cavity so that it melts. Then, the die is placedin the package cavity and pressed down on the preform and package sothat the die attach bond is made in a process called scratching. Afterthe die attach bond is made, the bond is cooled through natural heatdissipation during which no heat is supplied to the die attach bond.

U.S. Pat. Nos. 4,771,018 and 4,810,671 disclose an Au/Si die attach bondmethod that uses an Au/Si seed of a size that is approximately ten totwenty percent (10-20%) of the surface area of the die, in addition tousing an Au preform attached to the package cavity.

Another conventional die attach method is the silver/glass die attach.Silver/glass die attach can attach dies of larger sizes than the priorart Au/Si die attach methods. However, the silver/glass die attachmethod takes at least three hours for a pre- and post-curing process tomeet the thermal resistance and mechanical strength requirements of adie with 176K mil² area. The thermal resistance, θ_(jc), has beendetermined to be 0.9° C. /W in the silver glass die attach method for a176K mil² die, where θ_(jc) is defined by the equation: ##EQU1## P isthe power put into the device; T_(j) is the temperature at the junctionsurface of the die which is defined as the surface of the die where thecircuit is located; T_(c) is the temperature at the case. The case isthe bottom surface of the package when the package is oriented to beattached to a board ("live bug" situation). A low thermal resistance isdesired because a die attach bond with a low thermal resistance is ableto more effectively dissipate heat from the operating die.

It is an object of the present invention to have an Au/Si die attachmethod that can be used with large dies; as the term is used in thisapplication, large dies refer to dies with a size above 62.5K mil².Another object of the present invention is to have a die attach methodthat has a low thermal resistance, and a substantially void-free dieattach bond. Voids in the die attach bond causes a poor thermalresistance, θ_(jc), and may cause stress fractures.

SUMMARY OF THE INVENTION

One aspect of the invention is directed to a method for attaching largesilicon dies (up to 176K mil²) to ceramic packages. This method includespreheating the package to a temperature at which the Au/Si preformmelts, putting the Au/Si preform on the package so the Au/Si preformmelts, attaching the die to the package and gradually cooling thepackage with its attached die through each temperature in amonotonically decreasing sequence of temperatures and maintaining thepackage, die and preform at each of the temperatures in the sequence fora predetermined time period. The gradual cooling of the bond reduces thethermal resistance θ_(jc) of the bond, and also reduces stress fracturesand voids.

An additional aspect of the invention is directed to an apparatus forheating and controlling the temperature of a package when large dies areattached to packages with Au-Si preform. The apparatus includes aheating block having an array of segments having a beginning segment andan end segment arranged along a single line, wherein for any segment,another segment that is closer to the end segment than such segment is adownstream segment with respect to such segment. The heating block arrayincludes an attachment segment, the attachment segment divides the arrayinto a string of preheat segments including the beginning segment and astring of post-melting segments including the end segment.

The apparatus also includes means for heating the preheat segments inorder to preheat the package before attachment, and for heating theattachment segment to a predetermined temperature sufficient to melt theAu-Si preform on a preheated package when the package is in contact withthe attachment segment, and for heating each post-melting segment to atemperature higher than that of any post-melting segment downstream fromit in the array in order to gradually cool the package, the die and thepreform, thereby reducing voids and stress fractures in the preformconnecting a die to a package.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of the heating block, heating elements forheating the block and a power controller;

FIG. 2 is a perspective view of some of the preheating segments of theheating block with packages placed thereupon; and

FIG. 3 is a perspective view of some of the segments of the heatingblock, packages placed on the heating block segments, the Au/Si preformand the semiconductor die.

DETAILED DESCRIPTION

FIG. 1 is a schematic view of the heating block 1. The heating block isdivided into a number of segments 6, 10, 14, 5, 20, and 24. Each segmentis attached to one of the individual heating elements 4, 8, 12, 16, 18,22, which heats up a corresponding segment to a specific temperature.Additionally, adjacent segments are thermally insulated from each other.

Each of the heating elements 4, 8, 12, 16, 18, and 22 is connected tothe power controller 2. The desired temperature at each segment of theheating block is maintained using this power controller 2. The usercontrols the temperature of each segment on the heating block by settinga meter (not shown) on the power controller 2 that corresponds to thatsegment to the desired temperature. The power controller 2 monitors thecurrent temperature of each segment of the heating block and controlsthe temperature of each segment in a manner analogous to a householdthermostat. The power controller 2 turns on a heating element when thetemperature of the corresponding segment of the heating block is belowthe temperature set on the corresponding meter (not shown) of the powercontroller 2. When the heating element is on, the heating element heatsthe segment of the heating block so that the temperature of that segmentrises. When the current temperature of the segment rises above thedesired temperature, the power controller turns off the heating element.In this manner, the current temperature of each of the segments of theheating block is maintained at the desired temperature.

Furthermore, the heating elements may be each connected to a transformer(not shown), such as the commercially available "Variac" transformer. Inthat case, the power controller will turn each transformer on or off asdescribed above in order maintain the segment at the desiredtemperature.

The segments of the heating block are arranged in an array going fromthe beginning segment 6 to the end segment 24. Any segment that iscloser to the end segment 24 than a particular segment is defined to bea downstream segment with respect to that particular segment. Forexample, segment 20 is a downstream segment with respect to segment 10.The end segment 24 is therefore downstream to all other segments.

The preheat segments in string 3 are heated to temperatures in amonotonically increasing temperature sequence. Any preheat segment thatis a downstream segment with respect to a particular preheat segment instring 3 is heated to a higher temperature than that particular preheatsegment in the string. That is, segment 10 is of a higher temperaturethan segment 6, and segment 14 is of a higher temperature than segment10 and so on. In one embodiment of the present invention, there are fivepreheat segments in string 3.

The attachment segment 5 is the segment of the heating block where thedie is attached to the package. This segment is heated to a temperaturehigh enough to cause the Au/Si preform to melt in the package cavity.

The post-melting segments in string 7 are arranged after the attachmentsegment. The post-melting segments 7 follow a monotonically decreasingtemperature sequence. Any post-melting segment that is a downstreamsegment with respect to a particular post-melting segment is heated andmaintained at a lower temperature than that particular post-meltingsegment. For example, segment 24 is at a lower temperature than segment20. In one embodiment of the present invention, there are fivepost-melting segments in string 7.

In one embodiment of the present invention, the power controllercontrols the turning on and off of the heating elements so as tomaintain the segments of the heating block at the followingtemperatures:

    ______________________________________                                        Heating Block Segment                                                                          Temperature (T °C.)                                   ______________________________________                                        Preheat segment 1                                                                               80                                                          Preheat segment 2                                                                              160                                                          Preheat segment 3                                                                              245                                                          Preheat segment 4                                                                              327                                                          Preheat segment 5                                                                              421                                                          Attachment segment                                                                             490                                                          Post-melting segment 1                                                                         405                                                          Post-melting segment 2                                                                         322                                                          Post-melting segment 3                                                                         240                                                          Post-melting segment 4                                                                         160                                                          Post-melting segment 5                                                                          79                                                          ______________________________________                                    

The temperatures listed in the above table are the temperatures on theheating block. The temperature in the package cavity, of course, will belower. The temperature in the package cavity in the attachment segmentshould be 420°-423° C., for the die attach with the Au/Si preform. Thetemperatures of the heating block segments are stabilized at the abovetemperatures before the packages are placed upon the heating block.

FIG. 2 is a perspective view of a portion of the heating block 1 withfour packages 30 placed thereupon. The segments in FIG. 2 correspond tothe packages shown thereupon. The packages are manually loaded upon thepreheat segments until all the preheat segments contain packages. Oneway to do this is to place one of the packages into the first preheatsegment for a predetermined amount of time, and then move this packageinto the second segment put another package into the first segment, andkeep both of these packages on their respective segments for anotherpredetermined amount of time, and then repeat this process until apackage comes to the attachment segment and all the preheat segments areloaded with packages.

The preheating of the packages in the above manner by putting eachpackage in each of the preheat segments for a predetermined amount oftime and shifting to a downstream segment is advantageous since itfacilitates streamlined loading and unloading by the operators; however,it is not essential for the present invention. A package may bepreheated in some other manner, such as by first putting the package ina downstream preheat segment from the beginning segment thereby skippingsome of the preheat segments, or by putting the package directly in theattachment segment and just waiting until the package cavity reaches thetemperature required to melt the Au/Si preform before doing anotherstep.

Another way to load up the preheat segments with packages is to preloadthe preheat segments by placing a package on each of the preheatsegments. After preloading the preheat segments, a new package is placedin the beginning segment when all the packages are shifted, as describedbelow.

FIG. 3 is a perspective view that shows some of the heating blocks withpackages placed thereon. The packages in FIG. 3 are shown as pin gridarray packages, but the heating block can be used for attaching dies toother types of packages as well. When a package is manually placed intothe attachment segment 5, the Au/Si preform 38 is placed in the packagecavity 43. The Au/Si preform 38 consists of 98% wt Au and 2% wt Si,which is substantially an eutectic alloy of Au/Si. The eutectic point ofthe Au/Si alloy is 363° C., but in practical applications thetemperature required to melt the Au/Si preform is about 420° C.

Additionally, the size of the preform's surface area is about 75% to 80%of the size of the die's surface area. Matching the size of thepreform's surface area relative to the size of the die's surface area isimportant for the effectiveness of the present method. If the preform istoo small, the die bond created with the melted preform will not coverthe entire surface of the die. If the preform is too big, the meltedpreform is hard to control and the preform will spread outside thesurface of the die.

The Au/Si preform 38 melts in about a second. Next, the technician putsthe die 40 into the package cavity and scratches the die 40 upon themelted Au/Si preform and the package cavity 43 in order to attach thedie to the package. After a predetermined amount of time, the packagewith attached die along with all the other packages on the heating blockare manually moved into the next downstream segment. In this manner, apackage freshly loaded onto the beginning segment will spend thepredetermined amount of time in each of the preheat segments, afterwhich time it is then moved to the attachment segment, and spends thepredetermined amount of time at that segment while the die is beingattached to it. The package with the attached die will then be moved toeach of the post-melting segments and spend the predetermined amount oftime in each. When the package with the attached die reaches the endsegment, it is removed after it has spent the predetermined time periodthereon. The packages that are still on the heating block are then eachshifted one segment downstream and a new package is placed in thebeginning segment.

Prior art Au/Si die attach methods have not provided heat to the dieattach bond while the die attach bond is cooling, so voids and/or stressfractures were created in the die attach bond when a large die is beingattached. As discussed before, since each of the post-melting segmentsis maintained at a monotonically decreasing sequence of temperatures inthe downstream direction and the package with the attached die spends apredetermined amount of time in each post-melting segment, the dieattach bond formed between the package and the die cools graduallyrather than rapidly as in the prior art method. Due to the gradualcooling, the resulting die attach bond of the present invention issubstantially void free and has a low thermal resistivity. Voids mayproduce a stress difference that can cause stress fractures in the die.Additionally, air contained in the voids or fractures in the die attachbond makes a poor thermal conductor, so a void- and fracture-free dieattach bond has a lower thermal resistivity.

In the preferred embodiment of the present invention, the predeterminedamount of time a package spends in each segment is three minutes ormore. The thermal resistivity θ_(jc) of the die attach bond created bythe present die attach method has been measured as 0.6° C./W for a 176Kmil² die. This thermal resistance is lower than that of the silver/glassdie attach bond's thermal resistance of 0.9° C./W for a 176K mil² die.

Due to the lack of voids, low thermal resistance, and lack of stressfractures, the present invention's Au/Si die attach method can be usedfor large die sizes. The method has been designed for die sizes with anarea 300 mils×300 mils and die sizes up an area 550 mils×550 mils can beattached using the method of the present invention.

Various details of the implementation and method are merely illustrativeof the invention. It will be understood that various changes in suchdetails may be within the scope of the invention, which is to be limitedonly by the appended claims.

What is claimed is:
 1. A method for attaching a large die to a packagewith a layer of Au-Si preform comprising:preheating the package to atemperature at which the preforms would melt; placing the Au-Si preformon to the package so that the Au-Si preform melts; attaching the die onthe package through the preform to securely connect the die to thepackage by means of the preform after the preform cools; graduallycooling the package, die and preform sequentially through eachtemperature in a monotonically decreasing sequence of temperatures andmaintaining the package, die and preform substantially at each of saidtemperatures in the sequence for a predetermined time period so thatvoids and stress fractures in the preform are avoided.
 2. The method ofclaim 1, wherein said cooling is performed by periodically reducing theamount of heat supplied to the package, die and preform and maintainingthe amount of heat supplied at substantially constant levels for saidpredetermined time periods.
 3. The method of claim 1, wherein the stepof preheating the package comprises gradually heating the packagesequentially through each temperature in a monotonically increasingsequence of temperatures and maintaining the package, die and preformsubstantially at each of said temperatures in the sequence for apredetermined time period in order to heat the package so that differentparts of the package are heated to substantially the same temperaturesin the sequence.
 4. A method for attaching large dies to packages withan Au-Si preform by means of a heating block divided into an array ofsegments arranged along a single line and having a beginning segment andan end segment, wherein for any segment another segment that is closerto the end segment than such segment is a downstream segment withrespect to such segment, said array including an attachment segment,said attachment segment dividing the array into a string of preheatsegments including the beginning segment and a string of post-meltingsegments including the end segment, said preheat segments for preheatingthe package before attachment, and post-melting segments for heating thepackage and die after attachment, said method comprising the stepsof:(a) heating the array of segments to predetermined temperatures so asto preheat segments for preheating the packages, wherein the attachmentsegment is heated to a temperature sufficient to melt the Au-Si preform,wherein each post-melting segment is heated to a temperature higher thanany segment downstream from it in the array in order to gradually coolthe package, the die and the preform; (b) loading at least one of thepreheat segments with a package; (c) transferring the package from apreheat segment to a downstream preheat or attachment segment andrepeating said loading and transfer process in order to load andgradually preheat the packages, until a package lands on the attachmentsegment for heating such package to a temperature high enough to meltthe preform, such package being then ready for attachment; (d) placingthe Au-Si preform onto the package that is on the attachment segment andattaching the die to the package; (e) transferring each of the packagesin the array from one segment to a downstream segment and repeating saidtransfer process until a package and a die attached thereto lands on theend segment; and (f) removing the package and die from the end segmentof the heat block.
 5. The method of claim 4, wherein said loading steploads one package at a time onto the beginning segment.
 6. The method ofclaim 5, wherein the transferring steps in steps (c) and (e) eachtransfer a package from one segment to the adjacent downstream segmentin the array.
 7. The method of claim 4, wherein the steps (b)-(f) arerepeated.
 8. The method of claim 4, wherein the transferring steps insteps (c) and (e) occur after each package transferred spends three ormore minutes on the segment from which it is transferred.
 9. The methodof claim 4, wherein loading and transferring steps are such that eachpackage spends three or more minutes on each segment.
 10. The method ofclaim 4, wherein step (a) comprises heating each preheat segment in thepreheat segment string such that each segment is at a lower temperaturethan any segment downstream from it.
 11. The method of claim 9, whereinthe heating block comprises five preheat segments and five post-meltingsegments, and wherein step (a) comprises heating the preheat segments to80° C., 160° C., 245° C., 327° C., and 421° C. respectively, heating theattachment segment to 490° C., and heating the post-melting segments to405° C., 322° C., 240° C., 160° C., and 79° C., respectively.